Method of finding fingerprint appearance

Fingerprints can be divided into three categories:

The first category is obvious lines, which are visible lines. Such as paint, blood, ink, etc., are usually printed on fingerprint cards to become basic information;

The second type is molding pattern, which refers to fingerprints found on soft substances, such as candles and clay.

The third category is latent fingerprints, which are fingerprint lines formed by the transfer of natural body secretions, such as sweat.

It is not easy to find by eyes, and it is the most common fingerprint in the crime scene. Latent fingerprints are often left by fingers touching grease, sweat

liquid or dust before touching a clean surface. Although these fingerprints can't be seen by naked eyes, they can be revealed by special methods and using some special chemical reagents.

the fingerprint that forensic personnel most often come into contact with is latent print. If the fingerprint is left on the surface of non-absorbent items such as metal, plastic,

glass and ceramic tiles, the inspection method will be easier. Usually, powder method can be used to select powder with

large color contrast and scatter it on the surface of the article to extract a complete fingerprint; Another method is magnetic powder method, which uses fine < P > iron powder particles as a brush and brushes them back and forth to show fingerprints.

If fingerprints are left on the surface of absorbent articles such as paper, cards, leather and wood, they must be chemically treated

before they can be exposed in the laboratory. Commonly used chemical methods are:

Iodine fumigation-that is, iodine crystal is heated to generate steam, which reacts with the grease of fingerprint residue, and then

yellow-brown fingerprints will appear, which must be photographed immediately or fixed by chemical methods;

Ninhydrin method —— When the reagent is sprayed on the sample, it will have a purple fingerprint after it reacts with the amino acids in the body secretion.

silver nitrate method-after the silver nitrate solution reacts with the sodium amide in the latent fingerprint, a black

fingerprint will be produced in the sun;

Fluorescent reagent method-Fluorescent ammonia and o-phthalaldehyde react with protein or amino acid of fingerprint residue almost immediately,

resulting in a highly fluorescent fingerprint. This reagent can be used on the surface of colored articles.